The present invention relates to the art of diagnostic medical imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Commonly, in MRI, a substantially uniform temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged or examined is placed. Nuclei in the subject have spins which in the presence of the main magnetic field produce a net magnetization. The nuclei of the spin system precess in the magnetic field at the Larmor frequency, i.e., the resonant frequency. Radio frequency (RF) magnetic fields at and/or near the resonant frequency are used to manipulate the net magnetization of the spin system. Among other things, RF magnetic fields at the resonant frequency are used to, at least partially, tip the net magnetization from alignment with the main magnetic field into a plane transverse thereto. This is known as excitation, and the excited spins produce a magnetic field, at the resonant frequency, that is in turn observed by a receiver system. Shaped RF pulses applied in conjunction with gradient magnetic fields are used to manipulate magnetization in selected regions of the subject and produce a magnetic resonance (MR) signal. The resultant MR signal may be further manipulated through additional RF and/or gradient field manipulations to produce a series of echoes (i.e., an echo train) as the signal decays. The various echoes making up the MRI signal are typically encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data line""s position in k-space (i.e., its relative k-space row) is typically determined by its gradient encoding. Ultimately, in an imaging experiment, by employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space (or reciprocal space) data.
Fluid-attenuation inversion recovery (FLAIR) is a popular MRI technique employed to suppress unwanted signal from fluid near or around tissue that an operator wishes to visualize. It has been found particularly useful in brain scans and spinal imaging where brain tissue or spinal tissue is of interest and MR signal from surrounding cerebral spinal fluid (CSF) is undesirable. For example, FLAIR pulse sequences are commonly used to provide improved conspicuity of lesions located in regions of the body near CSF.
Where FLAIR is used to evaluate abnormalities in the brain and spine, suppression of the CSF in the images is commonly desired so that contrast differences in lesions, tumors, and edema in tissue proximal to the CSF will be enhanced. The application and timing of the inversion recovery (IR) RF pulses in MRI often dictate the type of contrast that is produced during a FLAIR acquisition. FLAIR sequences that apply selective IR RF pulses may, however, produce overly long acquisition times and may exhibit problematic in-flow artifacts such as those produced by CSF motion. This type of FLAIR is known as selective FLAIR.
As an alternative to selective FLAIR, non-selective (NS) FLAIR was developed. In NS-FLAIR, a single (or reduced number of) non-selective inversion pulse(s) that excites the entire region is applied before the acquisition pulses produced at each desired slice position (i.e., read out and turn on the data acquisition). Different tissue types (which have various relaxation characteristics) will produce different levels of signal amplitude depending on when the slices were acquired relative to the timing of the inversion recovery pulse. NS-FLAIR also reduces the CSF in-flow artifact because the inversion excites a large region. Since only a single inversion pulse is applied, NS-FLAIR sequences also provide faster acquisition times. However, imaging contrast is not as consistent through the slices when compared to selective IR excitation. The contrast in NS-FLAIR is typically dictated by the time when the slice was acquired relative to the NS inversion recovery RF pulse.
Moreover, in previously developed NS-FLAIR techniques, it has been suggested that the signed real values of images from two acquisitions (one with the original slice ordering and one with reverse slice order) are to be added together. In this way, the CSF signal amplitudes of each slice can be made more constant. However, this technique is less than optimal. Moreover, no explicit method for compensating for general phase differences between the images was considered, and the signal-to-noise ratio (SNR) has been shown to be worse when no phase correction is applied in connection with NS-FLAIR images.
The present invention contemplates a new and improved NS-FLAIR method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance imaging includes supporting a subject in an examination region of an MRI scanner, and setting up a spin system with a net magnetization in the subject. An RF inversion pulse is applied via the MRI scanner. The RF inversion pulse inverts the magnetization of the spin system in a selected volume of the subject. As the magnetization re-grows, a first set of raw data is generated by acquiring MR signals from a series of regions within the selected volume. For the first set of raw data, the series of regions are acquired in a first temporal order with respect to the RF inversion pulse. The RF inversion pulse is re-applied, and as the magnetization re-grows, a second set of raw data is generated by acquiring MR signals from the same series of regions. However, for the second set of raw data, the series of regions are acquired in a second temporal order with respect to the RF inversion pulse. The second temporal order is different from the first temporal order.
From the first and second sets of raw data, respectively, first and second sets of complex image data are generated. A complex phase correction factor is then determined and applied to one of, or both, the first and second sets of complex image data to thereby phase match the first and second sets of complex image data with one another. Ultimately, a combined image is generated. The combined image is generated via a pixel by pixel complex combination of the phase matched first and second sets of complex image data.
In accordance with another aspect of the present invention, a magnetic resonance imaging apparatus includes a main magnet that generates a substantially uniform temporally constant main magnetic field through an examination region wherein an object being imaged is positioned. A magnetic gradient generator produces magnetic gradients in the main magnetic field across the examination region, and a transmission system includes an RF transmitter that drives an RF coil which is proximate to the examination region. A sequence control manipulates the magnetic gradient generator and the transmission system to produce an NS-IR sequence. The NS-IR sequence induces detectable magnetic resonance signals from the object.
A reception system includes a receiver that receives and demodulates the magnetic resonance signals to obtain first and second sets of raw data. The first set of raw data represents a series regions from a selected volume of the object which was excited by an RF inversion pulse from the NS-IR sequence. The first series of regions are acquired in a first temporal order with respect to the RF inversion pulse. The second set of raw data represents the same series of regions, wherein the series of regions are acquired in a second temporal order which is different from the first temporal order.
The apparatus further includes a k-space storage device into which the first and second sets of raw data are rebinned as first and second sets of k-space data, respectively. A reconstruction processor subjects the first and second sets of k-space data to a reconstruction algorithm to generate first and second sets of complex image data, respectively, which are loaded into a complex image data storage device. A phase correction processor operates on one of, or both, the first and second sets of complex image data to thereby phase match the first and second sets of complex image data with one another. An image generator combines the phase matched first and second sets of complex image data into combined image data, and an output device produces human-viewable image representations of the object from data output by the image generator.
One advantage of the present invention is improved visualization of a selected tissue type (e.g., brain tissue or CSF) with suppression of the undesired tissue type.
Another advantage of the present invention is that tissue segmentation can be achieved thereby allowing tissue isolation and identification to aid in tissue specific processing of pixel data.
Another advantage of the present invention is optimized phase matching between data sets in an NS-FLAIR experiment.
Yet another advantage of the present invention is relatively shorter imaging time as compared to comparable selective FLAIR experiments.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.